662 S. TABER SOME CRITERIA USED IX RECOGNIZING FAULTS 



In regions where crustal movements are now going on with relative 

 rapidity, as on the Pacific coast, faults showing evidence of recent dis- 

 placement may be classed as active faults, and along them renewed ad- 

 justments may be expected to occur from time to time; but in regions 

 that are relatively stable the probability of displacements in the near 

 future is slight, even along recent faults, such as those of Mount Toby, 

 Massachusetts.^ 



Estimates of the relative stability of most regions have to be based, at 

 present, entirely on the magnitude of post-Pleistocene movements, and 

 on the seismic history over a period of no more than a few centuries at 

 most. Accordingly, accurate geodetic surveys should be made in the 

 more mobile portions of the earth, at such intervals as may be necessary, 

 in order to determine the character and rate of the crustal movements 

 now going on. Such surveys would probably give a better indication of 

 the stability of a region. 



Physiographic Evidence of recent Faulting 



Physiographic evidence is especially useful in locating active faults in 

 regions where earthquake records are available for only a short time. 

 Fresh fault-scarps, when present, indicate recent displacement, but scarps 

 may be masked under thick deposits of unconsolidated sediments, and 

 they are necessarily absent where the displacements are horizontal. Per- 

 haps the most characteristic evidence is furnished by the presence of 

 small depressions due to the downfaulting of narrow strips along the 

 strike of the fault, for they seem to be equally prevalent along faults 

 where the recent displacements have been horizontal and where they have 

 been dominantly vertical. 



The fault forming the Avest front of the Wasatch Range is marked by 

 a recent scarp having a maximum height of about 100 feet, which may 

 be traced interruptedly for a distance of about 125 miles. The facts 

 concerning the fault given in this paper have been abstracted partly 

 from Gilbert's monograph on Lake Bonneville- and partly from the field- 

 notes of the present writer. In places the scarp splits up into two or 

 more scarps, and where this branching is prominently developed narrow 

 fault troughs and small undrained basins are prevalent. The exposed 

 fault-planes dip toward the west at angles of 75 to 85 degrees. Striations 

 on boulders in cemented Bonneville gravels as well as on the Carbonifer- 



1 F. n. Looinis : Postglacial faulting about Mount Toby. Massachusotts. Rull. Geol. 

 Soc. Am., vol. 32, 1921, pp. 75-80. 



2 G. K. Gilbert : Lake Bonneville. U. S. Geol. Survey, Monograph I, 1890, pp. 340-.357, 



